The Bluff Hills

The Bluff Hills are the band of steep, deeply dissected loess country that borders the Mississippi River floodplain, and on the federal ecoregion map the Natchez Trace Parkway passes through a lobe of them for about a mile and a half between mileposts 28.5 and 30 in Jefferson County. On either side it runs in the Southern Rolling Plains, the gentler country it follows for most of the way from milepost 4 to Bayou Pierre. The two regions stand on the same older sands, silts and clays, among them the Catahoula Formation, under the same mantle of windblown silt. They differ in how much silt there is.

In the Bluff Hills the loess is often more than fifty feet thick, deeper and more calcareous than to the east, and it erodes into long, narrow, winding ridges and ravines, with steep streams and narrow floodplains. On the steepest of these soils slopes run from 12 to 90 percent, and the federal soil description calls erosion a severe hazard, with gullies cutting through unstable side slopes. Southern magnolia becomes more common on them than on the deep loess slopes further north. The parkway's 2016 vegetation mapping sampled a rare forest type at four sites, between mileposts 25 and 35 and near milepost 55: the Southern Loess Hills hardwood and pine forest, with sweetgum, southern magnolia, loblolly pine, water oak and cherrybark oak over silt loam. NatureServe ranks it between critically imperiled and imperiled worldwide, confined to the western Florida Parishes of Louisiana and the adjoining corner of southwest Mississippi, with most surviving stands on steep slopes and in ravines too hard to log or farm.

Eastward the mantle thins and turns more acid, stream gradients fall and floodplains widen, and pine is naturally more common, so that the historic forests of the Rolling Plains held shortleaf and loblolly pine among upland oaks and hickories. Both regions belong to the Southern Mississippi Valley Loess, a land resource area of about 26,520 square miles across seven states from Illinois to Louisiana. The parkway returns to the Bluff Hills north of Bayou Pierre, near milepost 50, and leaves them for the Loess Plains near milepost 59.